The aim of the research in this proposal is to understand the mechanisms of telomere function in eukaryotes. Telomeres are essential chromosomal elements which stabilize the ends of linear nuclear chromosomes and allow their complete replication. Much of the proposed work emphasizes the ciliated protozoans Tetrahymena and Euplotes, which, because of specific aspects of their biology, afford advantageous experimental systems for study of telomere, function. Telomere structure is highly conserved throughout eukaryotes, and findings made in one system are likely to be relevant to general questions of telomere function. Specific aims are to 1) determine the function of the essential RNA of a novel ribonucleoprotein enzyme, telomerase, that synthesizes the G-rich strand of the simple repeated sequences that form the essential cis-acting components of telomeres, 2) determine what structural features of telomeric DNA are required for recognition by telomerase, and investigate the mechanism of de novo telomere formation that occurs in ciliate macronuclear differentiation and chromosome healing, 3) investigate the enzymes responsible for synthesis of the complementary C-rich telomeric strand. 4) study the mechanism of genomic rearrangements in ciliate development, using an in vitro reaction system we have developed.